The present invention relates to integrated-circuit manufacturing and, more particularly, to oxide formations and pre-cleans therefor. A major objective of the present invention is to provide for more reliable and longer-lasting gate oxides.
Progress in integrated-circuit manufacture is generally associated with reductions in feature sizes; in fact, such progress can be roughly quantified as the minimum feature width definable by a given process technology. The features of interest are electrically active elements defined in a semiconductor substrate and conductive elements that provide access to and interconnection between the active elements. The minimum feature width for some of the earliest integrated circuits was a few microns; over the years, feature widths have dropped an order of magnitude. Reductions in feature width have allowed integrated circuit devices to be arranged closer together, providing greater functional density and higher operating speeds. Feature thickness has diminished along with feature width, although not as dramatically.
For large numbers of reliable integrated circuits to be manufactured, the structural and process materials involved must be precisely characterized. In general, this requires pure materials or pure materials that have been altered by the controlled introduction of impurities. While contamination is inevitable, even small amounts can impair the functioning, reliability, and/or lifetime of an integrated circuit. The susceptibility of integrated circuits to impairment by contamination has increased with decreasing feature sizes.
The gate oxide of a MOS transistor is an example of a feature that is highly susceptible to impairment by contamination. A MOS transistor typically comprises active elements including a source, a drain, and a channel. Current between the source and drain is controlled by the conductivity of the channel, which is, in turn, controlled by the voltage differential between the drain and a gate. The gate is a conductive element disposed over the channel and separated from it by the gate oxide. The gate oxide is intended to prevent current flow between the gate and the drain so that the gate-drain voltage required for channel control can be established.
If the gate-drain voltage is excessive, the gate oxide breaks down. When gate break-down occurs, current flows between the gate and drain, in which case, the transistor no longer functions as intended. Even one bad transistor can render useless an entire integrated circuit. The "breakdown" voltage at which such damage can occur depends on the thickness and purity of the gate oxide. As gate oxide thickness has decreased with generally decreasing feature sizes, the purity of the gate oxide (silicon dioxide) has become increasingly important.
Removal of contaminants generally involves dislodging and rinsing contaminants away. Several approaches to dislodging contaminants are available. Surface particles can often be dislodging by mechanically shaking a substrate, for example, using ultrasound. Detergents, such as ammonium hydroxide, can be used to weaken the bonds between particles and the substrate. Some contaminants can be dissolved and washed away in a solvent. Other contaminants can be chemically altered or combined so that they are soluble, for example, in water, for easy removal during a rinse. For example, hydrogen peroxide and ozone can be used to oxidize organic contaminants so that they are more soluble in water for removal.
For contaminants held fast in a silicon surface, an oxidizing agent can be used to convert silicon to silicon dioxide. An etchant, such as hydrogen fluoride, that etches silicon dioxide at a much greater rate than silicon can be used to etch the oxide growth; in the process, the entrained contaminants are dislodged from the substrate so that they can be removed in a rinse.
In accordance with these principles, a basic two-solution approach to contamination has been adopted by much of the semiconductor industry. The first solution includes hydrogen peroxide, and ammonium hydroxide in de-ioinized water. The ammonium hydroxide acts as a detergent, weakening the bonds between contamination particles and the substrate. The hydrogen peroxide is an oxidizing agent. It oxidizes organic contaminants so that they are water soluble. It also oxidizes substrate silicon. After a peroxide and ammonium hydroxide treatment, a rinse removes particles and oxidized organic material.
The second solution is hydrogen fluoride in de-ionized water. The hydrogen fluoride etches exposed silicon dioxide at roughly fifty times the rate it etches silicon. Thus, it removes existing silicon dioxide, including that grown in the presence of the peroxide in the first solution. Thus, the contaminants, entrained originally in the silicon surface and then in the oxide growth, are dislodged from the substrate for removal in a subsequent rinse. If further decontamination is desired, the two-solution treatments can be repeated. A drying step in isopropyl alcohol (IPA) vapor removes the final rinse water. A relatively pure silicon dioxide layer can then be grown or deposited.
The basic two-solution approach can be supplemented and modified in several ways. In some cases, one but not both of the solutions are used. Ozone is sometimes used instead of hydrogen peroxide as the oxiding agent. The bare silicon substrate can be cleaned ultrasonically before the basic two-solution approach is used. The basic approach can be preceded by a sulphuric acid treatment, for example, to remove remnants of photoresist. Various chelating agents, such as chloracetic acid and ethlenediamine-tetra-acetic acid (EDTA) that bind metal, or other chemical agents can be added to either solution to reduce the likelihood of recontamination by dislodged contaminants.
The various cleaning chemistries can be used alone or in sequence with others. Hydrogen fluoride is often a preferred last step because hydrogen fluoride can be obtained with relatively high purity, and the resulting surface is well suited for oxidization, chemical vapor deposition, and pre-metallization processes. In the course of its manufacture, a single integrated circuit can be subjected to more than ten cleaning steps; many of these can be variations of those described above. Despite the success of the cleaning chemistries developed to date, improved cleaning chemistries are desired to improve today's yields and reliability, and to permit even smaller devices in the future.